1. Field of the Invention
The present invention is generally in the field of data communications. More specifically, the present invention is in the field of modem communication over a telephone line.
2. Background Art
The increased demand and availability of a variety of data communications solutions has resulted in the use of different data communications devices and services, such as V.90 modems and ADSL services, over the same POTS (“plain old telephone service”) telephone line. By way of background, a typical POTS telephone line has an impedance in the range of approximately 600.0 to 900.0 ohms. The input impedance of a data communications device, such as a modem, connected to a POTS telephone line needs to closely match the impedance of the POTS telephone to meet minimum regulatory return loss requirements.
When a single data communications device, such as a V.90 modem, is connected to a POTS telephone line, the regulatory requirements can be met by making the input impedance of the V.90 modem approximately 600.0 ohms. However, if an ADSL device, such as an ADSL modem, is also connected, via a POTS splitter, to the same POTS telephone line as the V.90 modem, the resulting input impedance of the coupled data communications devices (i.e. the ADSL modem, V.90 modem, and POTS splitter) no longer matches the POTS telephone line impedance. In fact, the return loss of the above coupled communications devices falls below the regulatory minimum requirements.
FIG. 1 shows a block diagram of an exemplary system comprising a V.90 modem, a POTS splitter, and an ADSL load. In system 100 in FIG. 1, V.90 modem 102 is connected to POTS splitter 104 via lines 116 and 118. POTS splitter 104 can be a low pass filter. POTS splitter 104 is connected to TIP terminal 108 and RING terminal 110, respectively, via lines 112 and 114. TIP terminal 108 and RING terminal 110, respectively, are connected to the tip and ring terminals of a POTS telephone line. A first terminal of ADSL load 106 is connected to TIP terminal 108, and a second terminal of ADSL load 106 is connected to a first terminal of relay 120. A second terminal of relay 120 is connected to RING terminal 110. Thus, when relay 120 is closed, ADSL load 106 is connected in parallel with POTS splitter 104 at TIP terminal 108 and RING terminal 110. As an example, ADSL load 106 can be an ADSL modem. When system 100 is connected to a telephone line at TIP terminal 108 and RING terminal 110, the input impedance looking into line 112 must closely match the telephone line impedance for the return loss of system 100 to meet regulatory requirements.
When V.90 modem 102 is connected directly to a telephone line by itself, i.e. without either POTS splitter 104 or ADSL load 106 connected to the same telephone line, the input impedance of V.90 modem 102 must closely match the telephone line impedance for the return loss of V.90 modem 102 to meet regulatory requirements. This means that, for modems utilizing a Data Access Arrangement (“DAA”) circuit to interface with a telephone line, the input impedance of the DAA circuit must closely match the telephone line impedance. For example, to meet a desired telephone line impedance of 600.0 ohms, the DAA circuit inside V.90 modem 102 needs to have an input impedance that is close enough to 600.0 ohms to meet regulatory return loss requirements. However, in system 100, with POTS splitter 104, ADSL load 106, and V.90 modem 102 connected to the same telephone line, the combined impedance of POTS splitter 104, ADSL load 106, and V.90 modem 102 deviate from the telephone line impedance sufficiently such that the return loss of system 100 does not meet regulatory requirements. For example, FIG. 2 shows a graphical comparison of the simulated return loss of system 100, represented by waveform 204, and the minimum required return loss, represented by dashed line 202, for a 600.0 ohm impedance telephone line.
As seen in FIG. 2, waveform 204, representing the simulated return loss of system 100, falls below dashed line 202, representing the required return loss, in the 0.0 to 4.0 kHz frequency range, i.e. the frequency range where the return loss of system 100 is required to be above 20.0 dB. Thus, even when the input impedance of V.90 modem 102 can sufficiently match a telephone line impedance to enable V.90 modem 102 to meet regulatory return loss requirements, the input impedance of V.90 modem 102 combined with additional impedances of POTS splitter 104 and ADSL load 106 result in system 100 not meeting regulatory return loss requirements.
Thus, there is need in the art for impedance compensation to interface a modem to a telephone line in the presence of an external impedance load, such as an ADSL load, in order to meet regulatory return loss requirements.